Differentially coated galvanized strip



Nov. 26, 1963 G. LUSA DIFFERENTIALLY COATED GALVANIZED STRIP Filed Dec.28, 1959 rlo V INVENTOR GEORGE LUSA,

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ATTORNEYS United States Patent 3 112,213 DIFFERENTIALLY CATED GALVANIZEDSTRIP George Lusa, Dayton, Ohio, assignor to Armco Steel Corporation,Middletown, Ohio, a corporation of Qhio Filed Dec. 28, 1959, Ser. No.862,082 6 Claims. (Cl. 117-51) The invention relates to galvanized, i.e.zinc coated, ferrous strip or sheet materials bearing on one side astandard or heavy-weight zinc coating, and on the other side a coatingwhich, while derived from zinc, is very much thinner.

It has hitherto been realized that there are various uses for agalvanized product having a heavy or standard coatmg upon one side and alighter coating upon the other. Forexample, when galvanized sheets areused for roofing or siding on buildings, the heavy weights of coatingare needed on the exterior to secure a long life and resist corrosionwhen exposed to the weather; but the inside surface of such sheets, notbeing exposed to the elements, do not require such a heavy weight ofcoating.

Various suggest-ions have been made in the past looking toward theprovision of differentially coated galvanized sheets or strip. In thecoating operation, the ferrous material is passed through a bath ofmolten zinc and emerges therefrom between exit rolls which are groovedto control the extent and uniformity as well as the thickness of thecoating. It has been found that the exit roll on one side of the stripmay be grooved in such a way as to leave a lesser weight of coating onthe strip surface at that side. Again, it has been disclosed in thecopending application of Whitley, Kemplin and Jones, Serial No. 733,708,filed May 7, 1958, now Pat. No. 2,992,941, and entitled Exit Machine forCoating Apparatus and Method of Cont-rolling Coating Thickness, that theuse of an air blast on the meniscus at the exit rolls can decrease theamount of coating metal which is applied or left on the surface of thestrip. The meniscus, of course, refers to that quantity of zinc whichcollects above an exit roll and between it and the surface of the stripas it is leaving the exit roll.

It has also been found that the use of a wiper blade on one side of thestrip above a standard exit roll will serve to wipe off almost all ofthe coating resident on that side of the strip. The blade may be made ofmetal or any other substance of sufficient strength and capable ofwithstanding the heat involved.

One ditficulty with the formation of differentially coated galvanizedstrip lies in the fact that the wiped or otherwise more thinly coatedside of the material is likely to have a streaked and unsightlyappearance. While this may make no difference in certain uses, where thethinly coated side of the product is exposed to View, it will usually bedesired to have it covered with paint or enamel. Zinc coatings asusually produced are not very receptive to coatings of paint and enamelunless they are given pretreatmentsv These pretreatments are generallyexpensive and involve the installation of fairly bulky apparatus.

In still other uses, one side of the galvanized product may be given anorganic coating of paint, enamel or other material, while it isdesirable to have the other side carry a standard or heavy galvanizedcoating. If the organic coating can be made to adhere to a very thinzinc layer upon the one side of the product, it will obviously not benecessary for both sides of the product to carry heavy or standard zinccoatings. As an example of one such use, there are ceretain sheet metalparts of an automobile body, such as the so-called rocker panels, wherethe outer surface will be covered and protected with the ordinaryautomobile finish, but where the inner surface is exposed to air,moisture, and other corrosive influences but cannot readily be paintedor otherwise protected. The sheet 3,l l2,2l3 Patented Nov. 26, 1963 icemetal of this invention is useful for such parts. The external finish ofthe body will adhere very well to the thinly coated side of the productof the invention, which is located outwardly, while a full singlecoating on the inner side will protect that side from corrosiveinfluences.

Another problem is involved in the use of galvanized sheets on the undersides of automobiles. It is usual to spot weld adjacent panels together;but the standard zinc coatings cause a considerable amount of build-upon the copper welding electrode tips. This impedes the welding operationand makes for a frequent cessation of the Work so that the electrodetips may be cleaned.

The principal object of the invention is the provision of a newgalvanized product and a method of making it, which avoids all of thedifficulties mentioned above. This object, and others which will be setforth hereinafter or will be apparent to one skilled in the art uponreading these specifications, is accomplished by that procedure and inthat product of which certain exemplary embodiments will now bedescribed.

The drawing is a diagrammatic representation or flow sheet of one modeof carrying on the process.

The copending application of Schnedler, Serial No. 753,015, filed August4, 1958, now Pat. No. 2,986,808, teaches that a zinc coated sheet, whichhas been subjected to a heat treatment so as to cause an increase in thethickness of the interface alloy, will be usefully increased as to itsacceptance of paints and enamels while still having a surprising amountof corrosion resistance. However, it is not generally advantageous toincrease the extent of the interface alloy on both sides of the sheetfor reasons which will now be set forth:

The galvanized sheets are preferably formed from strip stock which hasbeen galvanized in accordance with the teachings of the Sendzimir U.S.Patent No. 2,197,622. In the practice taught by this patent, cold rolledstrip 1 is first passed through an oxidizing furnace 2 in which oils,greases, carbonaceous smudges, and other non-reducible materials areremoved from their surfaces, and a thin, controlled coating of oxide isformed thereon. The strip is next led into a reducing furnace 3 whereinthe thin coating of oxide is completely reduced. The temperature in thereducing furnace may, if desired, be sufficient to anneal the stock.From the reducing furnace, the strip passes through a cooling hood 4 inwhich it is still protected by reducing gases, and thence is carried asat 5 beneath the surface of a pot 6 of molten galvanizing metal 7 theentrance end of which is flux-free. The strip has a very short path oftravel through the galvanizing pot. Zinc metal in the pot contains asmall proportion of aluminum. All of these conditions combine to producea galvanized product in which the interface alloy is extremely thin. Itis believed to be due largely to this factor that the galvanized productis characterized by extraordinary ductility and adherence of thecoating, so that the coating can withstand substantially any bending,forming, drawing, or other working operations which can be performed onthe base stock. A heat treatment which would result in an increase inthe thickness of the interface alloy would, to a considerable extent,iznpair the ductility and adherence of the galvanized coating.

It has been found, however, in accordance with the present invention,that if one side of the galvanized strip is wiped shortly after itemerges from the exit rolls 8 and before the zinc coating thereon hashardened, so as to remove the greater part of the coating thereon, itthen becomes possible to apply a small amount of heat to the thinlycoated side of the strip so as to cause the zinc coating on that side toalloy rapidly with the base material, and that if the heat is properlycontrolled, the interface alloy on the opposite side of the strip willnot be increased in thickness, and that the standard or heavy Zinccoating thereon will not be affected as to its adherence or ductility.The new product, therefore, is a sheet or strip which has a standard orheavy zinc coating upon one side, the characteristics of which have notbeen affected, and upon the other side a very thin layer of zinc alloyedwith the ferrous base metal. Such a product has never before beenproduced.

In the practice of the invention the pretreatment of the strip and thecoating are carried on as is usual in the practice of the Sendzimirpatent to which reference has been made. If desired, the exit rolls maybe grooved in such a way as to apply coatings of different thicknessesto opposite sides of the strip, or an air blast may he used on themeniscus on one side, as mentioned above. These expedients, however, arenot necessary. The use of a scraper blade 9 is the preferred procedure,the blade being so located as to deflect the strip as shown and insurecontact all across its Width. A blade properly used will be effective inremoving the major portion of the zinc galvanizing metal on the side ofthe strip contacted by it.

The zinc coating on the opposite side of the strip does not constitute alimitation on the invention. Depending upon the usage to which theproduct is to be put, the heavy coating can be as heavy as that normallyproduced commercially, which could be as much as one ounce or heavier,per square foot. The other, or light side, the strip can have coatingsas light as .1 ounce, or less, per square foot.

The heat applied to the wiped side of the strip is conveniently producedby one or more gas ribbon burners It or some other suitable heat source,such as an electrical heating means. Assuming that the strip leaves thecoating pot in a vertical direction, the burners will be placed in ahorizontal position extending across the strip. The exact position ofthe burners with respect to the exit rolls is not critical; and it hasbeen found that the burners may vary from about one foot to ten feetfrom the exit rolls. The burners will be located at such a distance fromthe surface of the strip as to produce the proper heat at that surface.In general, the burners will lie from 2 inches to 8 inches away from thestrip. The heat may vary from 500 F. to about 1400 F. at the surface ofthe strip. The heating should be done in such a way that, while theadjacent and thinly coated surface of the strip is heated to the desiredtemperature, the rate of heat dissipation from the strip as it cools inair will prevent the transmission of any great degree of heat to theopposite side of the strip. The temperature may be varied in accordancewith the thickness of the zinc coating remaining on the wiped side ofthe strip, and the amount of heat required to cause it to alloy with thebase metal.

In the practice of the invention, it may be found advisable to raise thetemperature of the molten metal in the coating pot. The pot temperaturemay be raised as high as about 870 F. without essentially affecting thecharacteristics of the standard or heavy zinc coating upon the one sideof the sheet.

Modifications may be made in the invention without departing from thespirit of it. The invention having been described in certain exemplaryembodiments, what is claimed as new and desired to be secured by LettersPatent is:

1. A process of making a differentially coated galvanized stock whichcomprises forming zinc coatings of unequal thickness on opposite sidesof a ferrous sheet stock, and subjecting the sheet stock upon the sidehearing the coating of lesser thickness briefly to localized heatsufficient to alloy the zinc on the said last mentioned sidesubstantially entirely with the ferrous sheet stock, while leavingsubstantially unaffected on the opposite side of the sheet both thelayer of zinc coating metal thereon and any interface alloy existingbetween said layer and the base metal.

2. A process of producing a differentially coated galvanized ferroussheet product which comprises subjecting ferrous strip to a cleaning andoxidizing operation so as to form thereon a thin uniform film of oxide,passing the strip so treated through a reducing furnace and heatiug ittherein in the presence of reducing gases so as thoroughly to reduce thesaid oxide layer, and through a cooling hood containing reducing gasesso as to introduce it into a bath of molten zinc coating metal through aflux free surface thereof, withdrawing the said st." from said baththrough exit rolls, scraping one surface of said strip above one of saidexit rolls so as to remove the greater part of the coating of zinccoating metal thereon, and then subjecting the same side of said stripto localized heat to cause the remaining zinc thereon to alloy with theferrous strip surface without essentially affecting the coating of zinccoating metal or the opposite side of said strip.

3. The process claimed in claim 2 wherein said zinc coating metalcontains a minor percentage of aluminum.

A process of making a differentially coated galvanized stock whichcomprises passing ferrous strip through a bath of molten coating metalof which the primary constituent is zinc, under conditions to cause thesaid molten coating metal to cling to and coat the surfaces of the saidferrous strip, and upon withdrawal of the ferrous strip from the bath ofmolten coating metal treating the surfaces thereof so that the residentlayer of coating metal upon one side of it is substantially thicker thanthe coating thereof upon the other side, and after the said treatmentapplying localized heat to the thinner coating upon the last mentionedside of said ferrous strip so as to cause the said coating metal in thesaid thinner coating to alloy substantially entirely with the ferrousstrip, while leaving subtsantially unaffected upon the opposite side ofsaid strip both the thicker layer of coating metal thereon and anyinterface alloy existing between said layer and the base metal.

5. The process claimed in claim 4 wherein the coating metal is zinccontaining a minor percentage of aluminum.

6. The process claimed in claim 5 wherein the layer of coating metalupon the first mentioned side of said strip is at least about one ounceper square foot, and wherein the layer of coating metal as formed uponthe second mentioned side of said strip is not greater than about 0.1ounce per square foot of the strip surface.

References Cited in the file of this patent UNITED STATES PATENTS

2. A PROCESS OF PRODUCING A DIFFERENTIALLY COATED GALVANIZED FERROUSSHEET PRODUCT WHICH COMPRISES SUBJECTING FERROUS STRIP TO A CLEANING ANDOXIDIZING OPERATION SO AS TO FORM THEREON A THIN UNIFORM FILM OF OXIDE,PASSING THE STRIP SO TREATED THROUGH A REDUCING FURNACE AND HEATING ITTHEREIN IN THE PRESENCE OF REDUCING GASES SO AS THOROUGHLY TO REDUCE THESAID OXIDE LAYER, AND THROUGH A COOLING HOOD CONTAINING REDUCING GASESSO AS TO INTRODUCE IT INTO A BATH OF MOLTEN ZINC COATING METAL THROUGH AFLUX FREE SURFACE THEREOF, WITHDRAWING THE SAID STRIP FROM SAID BATHTHROUGH EXIT ROLLS. SCRAPING ONE SURFACE OF SAID STRIP ABOVE ONE OF SAIDEXIT ROLLS SO AS TO REMOVE THE GREATER PART OF THE COATING OF ZINCCOATING METAL THEREON, AND THEN SUBJECTING THE SAME SIDE OF SAID STRIPTO LOCALIZED HEAT TO CAUSE THE REMAINING ZINC THEREON TO ALLOY WITH THEFERROUS STRIP SURFACE WITHOUT ESSENTIALLY AFFECTING THE COATING OF ZINCCOATING METAL OR THE OPPOSITE SIDE OF SAID STRIP.